The densification of propellant (fuel, oxidizer, or monopropellant) by chilling is used in the launch-vehicle arts. Liquid oxygen (LOX) is one of the common propellants.
In the simplest type of propellant loading, a flow of cryogenic propellant is provided to the propellant tank of the vehicle. The propellant absorbs heat from the tank, the engine, and the fuel lines, and the heat so absorbed is rejected from the propellant in the tank by venting the tank, and allowing the propellant to boil. The continuous flow of cryogenic propellant tops off the tank, and the propellant in the tank remains at a mass density corresponding to the temperature at which the propellant boils.
An improved densification technique is described in U.S. Pat. No. 5,644,920, issued Jul. 8, 1997 in the name of Lak et al. The technique described therein densifies the propellant in order to allow the vehicle to carry more fuel mass in the same tank volume, while reducing the vapor pressure, and consequently lowering the tank operating pressure. The reduction of operating pressure allows the tank wall to be thinner than if the operating pressure were higher, and reduces the vehicle's mass. The reduction in vehicle mass, in turn, improves the payload capacity of the vehicle.
As described in the abovementioned Lak et al. patent, the cryogenic propellant can be supercooled well below the normal boiling point temperature by transporting the liquid propellant from the vehicle tanks to a ground-based cooling unit which uses a combination of a heat exchanger and compressor. The compressor lowers the coolant fluid bath pressure, resulting in a low-temperature boiling liquid in the heat exchanger, which is subsequently used to cool the propellant. The cooled propellant is returned to the vehicle propellant tank. As described in the Lak patent, the scheme described therein has the additional advantages of elimination of an on-board recirculation pump or bleed system, smaller and lighter engine pumps and valves, a more stable ullage gas, and reductions in tank fill operations.
Improved propellant densification is desired.